Power converter with automatic mode switching

ABSTRACT

A power converter is provided that has an alternating-current (AC) to direct-current (DC) switched-mode power converter circuit that converts alternating-current power into direct-current power for powering an attached electronic device. Power can be conserved by automatically placing the power converter circuit in a low-power standby mode of operation whenever the electronic device is detached from the power converter. A monitoring circuit can be powered by a capacitor or other energy storage element while the power converter is operating in the standby mode. If the monitoring circuit detects an output voltage change that is indicative of attachment of the electronic device or if the storage element needs to be replenished, the monitoring circuit can place the power converter circuit in an active mode of operation.

BACKGROUND

This relates to electronic devices and power converter circuits forelectronic devices.

Alternating current (AC) power is typically supplied from wall outletsand is sometimes referred to as line power. Electronic devices includecircuitry that runs from direct current (DC) power. Power convertercircuitry is used to convert AC power to DC power. The DC power that iscreated in this way may be used to power an electronic device. The DCpower that is created may also be used to charge a battery in anelectronic device.

In some applications, AC to DC power converter circuitry may beincorporated into an electronic device. For example, desktop computersoften include AC to DC power converter circuitry in the form of computerpower supply units. A computer power supply unit has a socket thatreceives an AC power cord. With this type of arrangement, the AC powercord may be plugged directly into the rear of the computer to supply ACpower without using an external power converter.

Although desktop computers are large enough to accommodate internalpower supplies, other devices such as handheld electronic devices andportable computers are not. As a result, typical handheld electronicdevices and laptop computers require the use of external powerconverters. When untethered from the power converter, a handheldelectronic device or portable computer may be powered by an internalbattery. When AC line power is available, the power converter is used toconvert AC power into DC power for the electronic device.

Compact AC-DC power converter designs are typically based onswitched-mode power supply architectures. Switched-mode power converterscontain switches such as transistor-based switches that work inconjunction with energy storage components such as inductive andcapacitive elements to regulate the production of DC power from an ACsource. A feedback path may be used to tap into the converter output andthereby ensure that a desired DC voltage level is produced under varyingloads.

High power converter efficiency is desirable for conserving power. Highpower conversion efficiency can be obtained by using efficient convertertopologies and low-loss parts. Even when an optimal design is used,however, there are residual power losses when operating a powerconverter. These residual losses are associated with leakage currentsand other parasitics that arise from running the switched-mode circuitryof the converter and lead to the consumption of power by the powerconverter even when the power converter is not being actively used topower an electronic device. Power consumption when the power converteris not being used to power an electronic device represents a source ofundesirable power loss that can be reduced without adversely affectingconverter functionality.

SUMMARY

A power converter may be provided that includes an energy storagecircuit. The power converter may receive an input signal such as a linepower signal and may produce a corresponding output signal such as apower signal for a device or other circuitry. The power converter may beplaced in standby mode to conserve power. In standby mode, the energystorage circuit may be used to power circuitry that can wake the powerconverter from standby when appropriate. The power converter circuit canbe provided as part of a stand-alone power adapter or may beincorporated into other electronic devices.

With one suitable arrangement, the power converter may be a powerconverter circuit such as an alternating-current (AC) to direct-current(DC) switched mode power converter circuit. The power converter circuitmay convert AC line power into DC power for powering an attachedelectronic device. For example, the power converter may be used forpowering an electronic device such as a cellular telephone, portablecomputer, or music player.

The power converter circuit may have a switch that is modulated tocontrol power flow. When the switch is turned off, the power convertercircuit is essentially shut down and will not produce a DC power at itsoutput. In this state, which is sometimes referred to as a standby modeor sleep mode, power consumption by the power converter is minimized.When it is desired to power an attached electronic device, the powerconverter circuit may operate in an active mode in which the switch isactively modulated to produce a desired output signal (e.g., the DCoutput voltage).

The power converter circuit may provide its output to an output linethrough switching circuitry. During normal operation, a monitor circuitplaces the switching circuitry in a closed state in which the powerconverter circuit is coupled to the output line and produces a DC outputvoltage for powering the electronic device. Periodically, the monitormay open the switching circuitry to isolate the power converter circuitfrom the output line. The behavior of the voltage on the output line canbe monitored by the monitor. In the presence of a load that draws power,the output line voltage will tend to sag. When driven by an internalboosting circuit with no load present, the output line voltage may rise(or may at least not fall past a given threshold). If the voltage on theoutput line rises (or does not fall past the given threshold), themonitor can conclude that the electronic device is detached from thepower converter. If the voltage on the output line falls (or falls pastthe given threshold), the monitor can conclude that the electronicdevice is attached to the power converter.

The power converter may include an energy storage element such as acapacitor or battery. When the power converter circuit is operating instandby mode, the monitor can draw power from the energy storageelement. This allows the monitor to actively monitor the state of theoutput line to automatically determine when an electronic device isreattached to the power converter. The monitor may also monitor thestatus of the energy storage element. If the energy storage elementbecomes depleted, the monitor can direct the power converter circuit tomomentarily transition from the standby mode of operation to the activemode of operation to replenish the energy storage element. If a drop inthe output line voltage is detected that is indicative of reattachmentof the electronic device to the power converter, the monitor mayactivate the power converter circuit so that the electronic device ispowered.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a system including a power converter andan electronic device in accordance with an embodiment of the presentinvention.

FIG. 2 is a circuit diagram showing illustrative components that may beused for a power converter of the type shown in the diagram of in FIG. 1in accordance with an embodiment of the present invention.

FIG. 3 is a graph showing how the output voltage from a power converterof the type shown in FIG. 2 may evolve when an electronic device isdetached from the power converter in accordance with an embodiment ofthe present invention.

FIG. 4 is a graph showing how the voltage on an energy storage elementin a power converter of the type shown in FIG. 2 may evolve duringstandby mode operations and energy replenishment operations inaccordance with an embodiment of the present invention.

FIG. 5 is a graph showing how the output voltage from a power converterof the type shown in FIG. 2 may evolve when an electronic device isattached to the power converter in accordance with an embodiment of thepresent invention.

FIG. 6 is a graph showing how the output voltage from a power converterof the type shown in FIG. 2 may evolve during a monitoring operation asan electronic device that is being powered by the power converterremains attached to the power converter in accordance with an embodimentof the present invention.

FIG. 7 is a diagram showing illustrative operating modes and operationsinvolved in transitioning between operating modes in a power converterof the type shown in FIG. 2 in accordance with an embodiment of thepresent invention.

FIG. 8 is a diagram of an electronic device showing how a monitorcircuit may be powered by an energy storage circuit to detect powersupply changes in a power supply line within the electronic device inaccordance with an embodiment of the present invention.

FIG. 9 is a diagram of an electronic device showing how a monitorcircuit that is powered by an energy storage circuit may be used to wakeup a power supply in the electronic device that has been placed instandby mode in accordance with an embodiment of the present invention.

FIG. 10 is a diagram of an electronic device having first and secondpower supplies showing how a monitor circuit that is powered by anenergy storage circuit may be used in controlling the first and secondpower supplies in accordance with an embodiment of the presentinvention.

FIG. 11 is a diagram of an illustrative power adapter housingconfiguration that may be used for power adapter circuitry of the typeshown in FIG. 1 in accordance with an embodiment of the presentinvention.

FIG. 12 is a diagram of an illustrative power adapter housingconfiguration that may be used for power adapter circuitry of the typeshown in FIG. 1 and that may have a magnetic attachment mechanism inaccordance with an embodiment of the present invention.

FIG. 13 is a diagram showing how a power converter circuit outputcapacitor may be used as an energy storage device to power a monitorcircuit in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION

Power converters, which are sometimes referred to as power adapters, areused to convert power levels and types. For example, a power convertermay be used to boost or reduce a direct-current (DC) power level. Powerconverters may also be used to convert alternating current (AC) powerinto DC power. Power converters that are used in converting AC power toDC power are sometimes described herein as an example. In general,however, power converter circuitry may include circuitry fortransforming any suitable input signal (e.g., AC or DC currents andvoltages) into any suitable output signal (e.g., boosted, reduced, orotherwise transformed AC or DC currents and voltages). The use of powerconverters such as AC-to-DC power converters that produce regulated DCoutput voltages from AC input signals is merely illustrative.

In a typical scenario, a power converter may be plugged into a source ofAC line power such as a wall outlet. The AC power source may providepower at 120 volts or 240 volts (as examples). Circuitry in the powerconverter may convert the AC line power that is received into DC power.For example, an AC to DC power converter may receive AC line power at aninput and may supply DC power at a corresponding output. The outputvoltage level may be 12 volts, 5 volts, or any other suitable DC outputlevel.

The circuitry in the power converter may be based on a switched modepower supply architecture. Switched mode power supplies use switchessuch as metal-oxide-semiconductor power transistors and associatedcontrol schemes such as pulse-width modulation control schemes orfrequency modulation control schemes to implement power conversionfunctions in relatively compact circuits. When the switching circuitryhas a first configuration, power is transferred from a power source to astorage element such as an inductor (e.g., a transformer) or acapacitor. When the switching circuitry has a second configuration,power is released from the storage element into a load. Feedback may beused to regulate the power transfer operation and thereby ensure thatthe output voltage is maintained at a desired level. Examples ofswitched mode power supply topologies that may be used in a powerconverter include buck converters, boost converters, flyback converters,etc.

With one suitable arrangement, which is described herein as an example,an AC to DC power converter may be implemented using a voltage rectifierand flyback converter. The voltage rectifier converts AC line power intoDC power at a relatively high voltage level. The flyback converterportion of the power converter steps down the DC power at the output ofthe rectifier circuit to 12 volts, 5 volts, or other suitably low levelfor operating circuitry in an electronic device. If desired, other powerconverter architectures may be used. The use of a switched mode powerconverter arrangement that is based on a flyback converter design isdescribed herein as an example.

An AC to DC power converter may supply DC power to any suitableelectronic device. Examples of an electronic device that may receive DCpower from an AC to DC power converter include a handheld computer, aminiature or wearable device, a portable computer, a desktop computer, arouter, an access point, a backup storage device with wirelesscommunications capabilities, a mobile telephone, a music player, aremote control, a global positioning system device, and a device thatcombines the functions of one or more of these devices. With onesuitable arrangement, which is sometimes described herein as an example,the electronic device that receives power from the AC to DC converter isa compact portable device such as a handheld electronic device (e.g., amobile telephone or music player). This is, however, merelyillustrative. The AC to DC power converter may be operated inconjunction with any suitable electronic device.

An illustrative system environment in which a power converter mayprovide power to an electronic device is shown in FIG. 1. As shown inFIG. 1, system 8 may include a source of AC power such as AC powersource 14, a power converter such as AC to DC power converter 12, and anelectronic device such as electronic device 10.

AC power source 14 may be, for example, a standard wall outlet thatsupplies AC line power via a power cord. Wall outlet power is typicallydelivered at AC voltages of about 110 volts to 240 volts.

Power converter 12 may include a power converter circuit such as AC-DCpower converter circuit 122. AC-DC power converter circuit 122 may bebased on a switched-mode power supply design such as a flyback converteror other suitable power converter topology.

Electronic device 10 may have a battery for use in powering device 10when unattached to power converter 12. When power converter 12 isplugged into AC power source 14 and when electronic device 10 isconnected to power converter 12, power converter 12 can transform ACpower that is received from AC power source 14 into DC power for device10.

If desired, connectors may be provided at the input and/or output ofpower converter 12. For example, device 10 may have a universal serialbus (USB) port into which a USB cable may be plugged. The USB cable maybe used to convey DC power between power converter 12 and electronicdevice 10. For example, the USB cable or other cable may contain a firstline such as positive power supply line 72 that is used to convey apositive DC voltage at 12 volts, 5 volts, or other suitable positive DCvoltage level from converter 12 to device 10. This DC voltage level issometimes referred to as Vbus and line 73 of converter 12 is sometimesreferred to as a power supply bus or output line. The USB cable or othercable may also have a second line such as ground line 74 that is used toconvey a ground voltage at 0 volts or other suitable ground voltagelevel to device 10. A cable such as a USB cable may also contain datalines that may optionally be used to convey information between device10 and converter 12.

When connected to power converter 12, electronic device 10 may receiveDC power through the power pins of the USB connector and cable (as anexample). The use of a USB connector to connect power converter 12 andelectronic device 10 is, however, merely illustrative. Any suitableplugs, jacks, ports, pins, other connectors, or a hardwired connectionmay be used to interconnect power converter 12 and electronic device 10if desired. Similarly, a hardwired connection or a suitable plug, jack,port, pin structure, or other connector may be used to connect powerconverter 12 to power source 14.

AC-DC power converter circuit 122 may convert AC power from AC source 14to DC power on output paths 64 and 70. Path 64 may be a positive powersupply line that is coupled to converter output line 73 via switch SW2.Path 70 may be a ground power supply line that is coupled to groundoutput 75 of converter 12 and ground line 74 in the cable or other pathconnecting converter 12 to device 10. Switching circuitry such as switchSW2 may be based on any suitable electrical components that can controlthe flow of DC power from the output of AC-DC power converter circuit122 to the power supply input lines associated with electronic device 10(e.g., the inputs of device 10 that are connected to power supply lines72 and 74). For example, switching circuitry SW2 may be implementedusing one or more transistors such as one or more power field-effecttransistors (power FETs). During normal operation in which an electronicdevice such as electronic device 10 is connected to power converter 12,power converter 12 may use AC-DC power converter circuit 122 to supply aDC power supply voltage on lines 64 and 70. Switching circuitry SW2 willgenerally be closed during normal operation, so line 64 will be shortedto output line 73. This allows the DC power supply voltages at theoutput of AC-DC power converter circuit 122 to be provided to electronicdevice via paths 72 and 74.

AC-DC power converter circuit 122 may contain control circuitry forcontrolling internal switching circuits. The control circuitry may beresponsive to feedback signals. For example, a feedback path may be usedto supply AC-DC power converter circuit 122 with information on thecurrent level of voltage Vbus on output line 73. In response to thisfeedback information, the control circuitry in AC-DC power convertercircuit 122 can make real-time adjustments to the amount of DC voltagethat is being supplied to the output of AC-DC power converter circuit.For example, if the DC voltage on output 64 has a nominal value Vsec of5 volts and feedback indicates that the voltage has undesirably risen to5.05 volts, the control circuitry in AC-DC power converter circuit 122can make adjustments to lower the DC output voltage back to the nominalvalue (Vsec).

Power converter 12 may contain an energy storage circuit 50. Energystorage circuit 50 (sometimes also referred to as an energy storageelement) may be based on any suitable circuitry for storing energy. Asan example, energy storage circuit 50 may include one or more batteries,capacitors, etc. During operation of power converter 12 when AC-DC powerconverter circuit 122 is supplying power to output path 64, a path suchas path 66 may be used to route power to energy storage circuit 50. Thepower that is routed to energy storage circuit 50 in this way may beused to replenish the battery, capacitor or other energy storagecomponents in circuit 50. In the example of FIG. 1, energy storagecircuit 50 is coupled to AC-DC power converter circuit 122 by paths 64and 66. This is, however, merely illustrative. Any suitable routingpaths may be used to supply replenishing power from AC-DC powerconverter circuit 122 to energy storage circuit 50 if desired.

As shown in FIG. 1, power converter 12 may include monitoring circuitrysuch as monitor 54. Monitor 54 may monitor the status of power converter12 using paths such as paths 66 and 60. When appropriate, monitor 54 mayprovide control signals to AC-DC power converter circuit 122 using pathssuch as path 76. The control signals may be used to place AC-DC powerconverter circuit in an appropriate operating mode. In general, anysuitable number of operating modes may be supported by AC-DC powerconverter circuit 122 if desired.

With one suitable arrangement, which is sometimes described herein as anexample, AC-DC power converter circuit 122 may be placed in an activemode and a standby mode. In the active mode, which is sometimes alsoreferred to as a high-power mode or normal operating mode, AC-DC powerconverter 122 is on and supplies DC output power for replenishing energystorage circuit 50 and for powering electronic device 10. In the standbymode, which is sometimes referred to as a sleep mode or low-power mode,AC-DC power converter circuit 122 is placed in a state in which littleor no power is consumed by AC-DC power converter circuit 122 (i.e.,AC-DC power converter circuit 122 is turned off by inhibiting modulationof its switched-mode power supply switches). If desired, AC-DC powerconverter circuit 122 may have multiple lower power states (e.g., apartly off state and a fully-off state). Arrangements in which AC-DCpower converter 122 is placed in either a standby state or an activestate are sometimes described herein as an example. This is, however,merely illustrative. Power converter 12 may, in general, support anysuitable number of operating modes (e.g., a fully-on mode, a partly-onmode, a sleep mode, a deep sleep mode, etc.).

When AC-DC power converter circuit 122 is in standby mode, AC-DC powerconverter circuit 122 is off and allows output 64 to float. In thissituation, the power that has been stored in energy storage circuit 50may be delivered to path 66 from within energy storage circuit 50. Forexample, if energy storage circuit 50 contains a battery or a capacitor,the battery or capacitor may be used to supply a battery or capacitorvoltage to path 66. The voltage supplied by energy storage circuit 50may be supplied at the same voltage level as the nominal output voltagelevel (Vsec) that AC-DC power converter circuit 122 supplies to path 64when AC-DC power converter circuit 122 is in active mode.

Voltage regulator 66 may receive the voltage supplied by energy storagecircuit 50 via path 66 on its input IN and may supply a correspondingoutput voltage to output path 58 via its output OUT. In the absence of aload on output line 73, the voltage that voltage regulator 52 suppliesto path 58 may be elevated with respect to Vsec (i.e., the voltagesupplied by voltage regulator 52 to path 58 during standby operationsmay be equal to an elevated voltage Vaux that is larger than Vsec). If,for example, Vsec is 5.0 volts (as an example), Vaux may be 5.1 volts(as an example).

Output line 58 may be coupled to output line 73 and path 72 by path 56.During standby mode, monitor 54 may supply a switch control signal toswitching circuitry SW2 via a path such as path 62. The control signalmay place SW2 in an open mode in which lines 64 and 73 are electricallydisconnected from each other. Disconnecting output line 73 from path 64isolates output 73 from AC-DC power converter circuit 122 and energystorage circuit 50. The voltage that output line 73 assumes followingthe opening of switching circuitry SW2 by monitor 54 depends on thestatus of electronic device 10.

If electronic device 10 is disconnected from power converter 12 whenswitching circuitry SW2 opens, voltage regulator 52 will supply elevatedvoltage Vaux to output line 73 via paths 58 and 56, thereby driving Vbusto Vaux. If electronic device 10 is connected to power converter 12 whenmonitor 54 opens switching circuitry SW2, electronic device 10 willoperate as a load and will draw power from voltage regulator output OUTvia lines 58 and 56. Voltage regulator 52 may contain a current limitingcircuit that ensures that voltage regulator 52 will only be able tosupply a relatively modest amount of current to electronic device 10. Asa result, the power draw from electronic device 10 will pull Vbus low.

Monitor 54 may determine the attachment status of electronic device 10by monitoring the voltage Vbus on output line 73 via paths 56 and 60. Ifthe monitor detects a rise in voltage Vbus when switching circuitry SW2is opened, monitor 54 can conclude that electronic device 10 iscurrently detached from power converter 12. If monitor 54 detects a dropin voltage Vbus when switching circuitry SW2 is opened, monitor 54 canconclude that electronic device 10 is currently attached to powerconverter 12. Whenever monitor 54 determines that electronic device 10is attached to power converter 12, monitor 54 may place AC-DC powerconverter circuit 122 in active mode to supply device 10 with power. Ifthe presence of electronic device 10 is not detected, monitor may leaveAC-DC power converter circuit in standby mode to conserve power. Ifmonitor 54 detects that energy storage circuit 50 has become depleteddue to prolonged operation in standby mode, monitor 54 may awaken AC-DCpower converter circuit 122 momentarily to replenish energy storagecircuit 50.

Power converter 12 of FIG. 1 may be implemented using any suitablecircuits. Illustrative circuitry that may be used in implementing powerconverter 12 is shown in FIG. 2. In the example of FIG. 2, powerconverter circuit 122 has been formed using a flyback switched-modepower supply design. This is, however, merely illustrative. Any suitablepower converter circuitry may be used for AC-DC power converter circuit122 if desired.

As shown in FIG. 2, AC source 14 may be coupled to power converter 12 atterminals L and N. AC power from terminals L and N may be supplied topaths 20 and 22.

Power converter 12 may have rectifier circuitry 16. Diodes 18 mayconvert AC voltages on paths 20 and 22 to rectified (positive) signalsacross lines 24 and 26. The AC voltage on paths 20 and 22 may besinusoidal and the output of rectifier circuit 16 may be a rectifiedsinusoid. To smooth out the raw rectified output from diodes 18, powerconverter 12 may include capacitor 28. Capacitor 28, which may beconsidered to be part of rectifier 16, converts the rectified version ofthe AC signal from source 14 into a DC voltage on node 30 with a reducedamount of AC ripple.

AC-DC power converter circuit 122 may include a power converter controlcircuit such as converter control circuit 38. Ground line 56 may be usedto connect converter control circuit to ground path 24. Positive powersupply voltage Vb may be supplied to converter control circuit 38 atinput 84. Input 84 may be provided with voltage Vb by tapping powersupply line 26 using bleed circuit 82. Bleed circuit 82 may containcurrent limiting components such as one or more resistors.

Transformer 32 may have an input connected to the output of rectifier 16and an output connected to diode 40 and capacitor 42. Transformer 32 mayhave a turn ratio such as a 10:1 or 20:1 turn ratio. Switching circuitrySW1 such as a bipolar or metal-oxide-semiconductor power transistor maybe used to regulate the current Ip that flows through the primary sideof transformer 32. Switch SW1 may receive a control signal on controlinput 36 from converter control circuit 38. The control signal may havea frequency of about 20 kHz to 100 kHz (as an example). Control circuit38 may produce the control signal on line 36 to regulate the flow ofpower through converter 12. When power converter 12 is operated inactive mode, the control signal is active and is changed as needed toregulate the magnitude of voltage Vbus. When power converter 12 is instandby mode, the control signal is inactive (i.e., there is notime-varying control signal present on line 36). This reduces powerconsumption in power converter 12 that would otherwise arise from theoperation of switching circuitry SW1, even in the absence of a connectedload on output line 73. Standby power consumption can be further reducedby opening optional switching circuitry such as switches SW3 and SW4 toreduce leakage currents (e.g., using control signals from convertercontrol circuit 38 and/or from monitor 54).

The control signal that is provided to switching circuitry SW1 on line36 may be a signal whose frequency is adjusted to control the amount ofpower that flows through the converter or may be a signal such as apulse width modulation (PWM) signal whose duty cycle is adjusted tocontrol the amount of power that flows through the converter inaccordance with a pulse width modulation scheme.

With a typical PWM scheme, the control signal on line 36 may have a highvalue when it is desired to turn switch SW1 on to permit current Ip toflow and may have a low value when it is desired to turn switch SW1 offto prevent current Ip from flowing. The control signal on line 36 may,for example, be a square wave PWM signal whose duty cycle may beregulated by control circuit 38 to adjust the magnitude of Vbus onoutput 73. If desired, a frequency modulation scheme may be used. In afrequency modulation scheme, the control signal on line 36 may be asquare wave or other control signal whose frequency is regulated bycontrol circuit 38 to adjust the magnitude of voltage Vbus. The use ofPWM control signals in power converters such as power converter 12 issometimes described herein as an example. The use of PWM control signalsis, however, merely illustrative. Any suitable type of control signalmay be used to control power flow in converter 12 if desired.

When control circuit 38 applies a control signal such as a PWM controlsignal to switch SW1, the current Is at the secondary side oftransformer 32 will have a frequency equal to that of the control signal(e.g., about 20 kHz to 100 kHz). Diode 40 and capacitor 42 convert thisAC signal into a DC voltage at node 44. This voltage is provided to line64 and represents the output of AC-DC power converter circuit 122 ofFIG. 1. The nominal power supply output voltage on line 64, which issometimes referred to herein as Vsec, may be, for example, 12 volts, 5volts, or other suitable voltage. When electronic device 10 is connectedto output line 73 during active mode, the voltage that is produced atoutput 64 may be routed to electronic device 10 through switchingcircuitry SW2, output 73, and path 72 to power the circuitry ofelectronic device 10.

Power converter 12 may be controlled using an open-loop control scheme.With this type of arrangement, power converter 12 can apply apredetermined PWM signal, frequency modulation signal, or other controlsignal to switching circuitry SW1 to produce a desired output level onoutput 64 and output line 73. If desired, a closed-loop control schememay be used by providing a feedback path FB such as feedback path formedfrom lines 48 and 49. Using lines 48 and 49, control circuit 38 canreceive feedback on the current voltage level across nodes 44 and 46(i.e., the output voltage on line 64). If the currently monitored valueof the output voltage on node 44 is below a desired target level (i.e.,below the desired Vsec level), the duty cycle of the PWM signal or, in afrequency modulation scheme, the frequency of the control signal can beincreased to increase the output voltage accordingly. If control circuit38 determines that the output voltage on node 44 and output 64 of AC-DCpower converter circuit 122 is too high, the duty cycle of the PWMsignal or the frequency of the control signal can be decreased to reducethe output voltage towards its desired target level.

Circuitry such as converter control circuit 38 may be located on theprimary side of transformer 32. Circuitry such as monitor circuitry 54,energy storage element 50, switching circuitry SW2, and voltageregulator 52 may be located on the secondary side of transformer 32. Ifdesired, an isolation stage such as isolation stage 51 may be includedin feedback path FB to help electrically isolate circuitry on theprimary and secondary sides of transformer 32. Similarly, an isolationstage such as isolation stage 78 may be included in control path 76between monitor 54 and converter control circuit 38. Isolation stages 51and 78 may be formed from signal transformers, optical isolationdevices, etc.

As shown in FIG. 2, energy storage circuit 50 may be formed from anenergy storage element such as capacitor 80. Capacitor 80 may be coupledbetween path 66 and ground (e.g., node 46). During standby operations,capacitor 80 may be used to power monitor 54 and voltage regulator 52.Monitor 54 can monitor the output voltage from capacitor 80 on path 66to determine when capacitor 80 has become depleted enough to warrantreplenishment. When replenishment of the energy of capacitor 80 isdesired, monitor 54 can issue a wake-up control signal to convertercontrol circuit 38 via control path 76. In response, converter controlcircuit 38 can transition to active mode by resuming the generation ofcontrol signals on control line 36. This will lead to the production ofa DC output voltage on line 64 that can be routed to capacitor 80 viapath 66 to recharge capacitor 80. Battery-based energy storage elementsmay also be recharged in this way when they become depleted. An energystorage element 50 that is based on a battery may have, for example, acharger circuit connected between path 66 and the battery.

Voltage regulator circuit 52 may be formed from a DC-DC power convertersuch as a DC-DC boost converter 52A and a current limiting circuit suchas current limiting circuit 52B. If desired, the current limitingcapabilities of current limiting circuit 52B may be combined with thevoltage regulating capabilities of power converter circuit 52A. In theexample of FIG. 2, voltage regulation and current limiting functionshave been implemented using separate circuits. This is merelyillustrative. Circuits such as power converter 52A and 52B may be formedfrom one, two, or more than two integrated circuits and may, if desired,include discrete components.

Power converter 52A may be a switched-mode power supply such as a boostcircuit formed from control circuitry (such as control circuit 38),storage elements (capacitors and/or inductors), and other components(e.g., diodes). Electrical components such as these may be implementedas part of a single integrated circuit. During operation, boostconverter 52A may receive power (e.g., a DC voltage Vstore fromcapacitor 80) on input IN and may provide a corresponding output voltageon output OUT. The output voltage on output OUT of power converter 52Amay be lower or higher than the voltage Vstore. In the example of FIG.2, converter 52A is a boost converter that produces a nominal outputvoltage Vaux on output OUT that is greater than the nominal outputvoltage Vsec produced at output 64 of power converter circuit 122. If,for example, Vsec is 5.0 volts, Vaux may be 5.1 volts (as an example).The voltage Vstore may range from 5.0 volts when capacitor 80 is fullycharged to a lower value (e.g., a voltage in the range of about 1-4.5volts) as capacitor 80 becomes depleted.

Current limiting circuit 52B may be implemented using one or moreresistors or other suitable circuitry for limiting the maximum amount ofcurrent that may be drawn from power converter 52A when a load isconnected to output line 73.

When power converter circuit 122 is in standby mode, switch SW2 will beopen. In the absence of a load on output line 73, current limitingcircuit 52B can pass the voltage on output OUT of boost converter 52A toline 58 with negligible change in magnitude. In this situation, if thenominal output voltage from boost converter 52A is Vaux, the DC voltageVbus on output line 73 will rise to Vaux.

When a load such as electronic device 10 is connected to power converter12, the voltage Vbus on output line 73 will be pulled low. Boostconverter 52A will not be able to maintain Vbus at Vaux in thissituation, because current limiting circuit 52B serves to limit theamount of current that can be supplied to device 10. This causes voltageVbus to sag under load.

Monitor 54 can therefore monitor the attachment status of electronicdevice 10 by measuring the voltage Vbus and observing changes that takeplace in Vbus while controlling switching circuitry SW2.

FIG. 3 shows how the voltage Vbus on output line 73 may evolve when auser detaches electronic device from power converter 12. At times beforet0, electronic device 10 is attached to power converter 12 and receivesDC power over lines 72 and 74. Power converter circuitry 122 is in itsactive mode and supplies a DC output voltage at nominal output voltageVsec on output 64. Switching circuitry SW2 is closed during active mode,so the voltage Vsec on output 64 of power converter circuit 122 ispassed to power converter output line 73. The voltage Vbus on line 73 istherefore equal to Vsec at times before t0. At time t0, the userdetaches electronic device 10 from output line 73. Because output line73 is connected to output 64, which is supplying voltage Vsec, voltageVbus remains at voltage Vsec. At time t1, monitor 54 opens switchingcircuitry SW2 to isolate output line 73 from power converter circuit122. Monitor 54 may open switching circuitry SW2 in this way once everyfew seconds or minutes or at other suitable times to check theattachment status of electronic device 10.

At times after time t0, electronic device 10 is no longer attached tooutput line 73. As a result, when switching circuitry SW2 is opened attime t1, electronic device 10 no longer supplies a load to output line73. This allows Vbus to rise to the level of voltage Vaux that issupplied at output OUT of voltage regulator 52, as indicated by slopingsegment 87 of curve 86. Monitor 54 can monitor this rise in voltage Vbususing path 60. When a predefined threshold voltage such as thresholdvoltage Vth2 is reached at time t2, monitor 54 can conclude thatelectronic device 10 has been removed from power converter 12. Monitor54 can therefore issue a power-down command to power converter circuit122 over control path 76 to place AC-DC power converter circuit 122 andpower converter 12 into a standby power consumption mode. In this mode,switching circuit SW2 remains open, so voltage Vbus may rise to Vaux attimes t between times t2 and t3.

Line 88 in the graph of FIG. 4 illustrates how the voltage Vstore onpath 66 at the output of capacitor 80 may evolve as a function of timewhen electronic device 10 is detached from power converter 12. At timeti, power converter 12 is in standby mode. In standby mode, powerconverter circuit 122 is off (i.e., not actively switching switch SW1)and monitor 54 is being powered by stored energy in capacitor 80.Initially, at time ti, capacitor 80 has a voltage Vstore of Vsec (i.e.,the nominal output voltage on output 64 that is produced by powerconverter circuit 122 when power converter circuit 122 is active).

During times ti to td, monitor 54 operates to detect changes in theattachment status of electronic device 10. This consumes power anddepletes capacitor 80, leading to the decrease in voltage Vstore fromVsec to Vth4, as indicated by curve segment 90.

At time td, Vstore drops below a predetermined threshold voltage Vth4.When monitor 54 detects that Vstore has dropped below Vth4, monitor 54may issue an activation control command on path 76 that turns on powerconverter circuit 122. Once power converter circuit 122 is placed inactive mode at time td, the output voltage on output 64 will rise tonominal output value Vsec, as indicated by line segment 92 of curve 88.

Monitor 54 can monitor the replenishment process represented by linesegment 92 to confirm when Vstore has returned to its fully replenishedstate or can direct power converter circuit 122 to remain active for agiven period of time (e.g., a period such as a few seconds that issufficient to recharge capacitor 80). At time tr, after capacitor 80 hasbeen replenished, monitor 54 can place power converter circuit 122 instandby mode. As indicated by line segment 94, the depletion process ofline segment 90 then repeats. Monitor 54 can turn power convertercircuit 122 on and off as shown in FIG. 4 for as long as required (i.e.,until electronic device 10 is attached).

The graph of FIG. 5 illustrates how voltage Vbus may evolve during theprocess of attaching electronic device 10 to power converter 12. At timets, electronic device 10 is not attached to power converter 12. In theabsence of a load on output line 73, the voltage Vbus rises to Vaux tomatch the unloaded output voltage of voltage regulator 54, as shown byline segment 98 of curve 96. At time ta, a user attaches electronicdevice 10 to power converter 12 (e.g., by connecting a USB cable orother cable between device 10 and power converter 12). Once device 10 isconnected to output line 73, device 10 begins to load output line 73.

Current limiting circuit 52B prevents voltage regulator 52 fromproviding the full amount of current that is demanded by electronicdevice 10. This causes voltage Vbus to drop from Vaux at time ta to apredetermined threshold voltage such as Vth3 at time tb, as indicated byline segment 100. When monitor 54 detects that voltage Vbus has droppedto Vth3, monitor 54 can conclude that electronic device 10 has beenattached to power converter 12. Monitor 54 can therefore issue a commandto power converter circuit 122 over path 76 that places power convertercircuit 122 in its active mode.

Once power converter circuit 122 is activated, the output voltage frompower converter circuit 122 can supply power to electronic device 10,allowing voltage Vbus to rise to its nominal value Vsec, as indicated byline segment 102 in FIG. 5. At times after time tc (e.g., along linesegment 104), Vbus may be held at voltage Vsec by converter controlcircuit 38.

FIG. 6 shows how Vbus may evolve when monitor 54 opens switchingcircuitry SW2 while electronic device 10 remains attached. At time tbg,power converter 12 is active and is powering electronic device 10 bysupplying a voltage Vbus of Vsec. At time top, monitor 54 opensswitching circuitry SW2. Because electronic device 10 is connected topower converter 12, voltage Vbus drops. When a predetermined thresholdvoltage Vth1 is reached at time tcl, monitor 54 can conclude thatelectronic device 10 is still connected to power converter 12 and canclose switching circuitry SW2. Voltage Vbus preferably remains abovevoltage Vmin (e.g., about 4.5 volts) to prevent electronic device 10from erroneously concluding that electronic device 10 has beendisconnected from power converter 12. Once switching circuitry SW2 isclosed, power is restored to output line 73 and voltage Vbus will rise,reaching nominal output voltage level Vsec at time tfn.

A diagram showing how power converter 12 and device 10 may operate insystem 8 of FIG. 1 as a user attaches and detaches device 10 fromconverter 12 is shown in FIG. 7.

In active mode 106, power converter 12 is operating normally as an AC-DCpower converter and is supplying power to an attached electronic device10 from AC source 14. In a typical scenario, electronic device 10contains a rechargeable battery that can be recharged when electronicdevice 10 is connected to power converter 12. During the operations ofmode 106, monitor 54 primarily holds switching circuitry SW2 closed toallow power to be delivered from line 64 to output line 73 andelectronic device 10. At appropriate times (e.g., once every fewseconds, minutes, etc.), monitor 54 momentarily opens switchingcircuitry SW2 to check whether electronic device 10 is still attached.If voltage Vbus does not rise when switching circuitry SW2 is opened(e.g., if voltage Vbus falls to Vth1 as described in connection withFIG. 6), monitor 54 can conclude that electronic device 10 is stillattached to power converter 12. As indicated by line 108, the operationsof active mode 106 can then continue uninterrupted.

If, however, voltage Vbus rises to threshold Vth2 when switchingcircuitry SW2 is opened as described in connection with FIG. 3, monitor54 can conclude that device 10 has been detached. As shown by line 110,monitor 54 can then place power converter circuit 122 and powerconverter 12 in standby mode 114.

During standby mode 114, power converter circuit 122 is not active, sopower converter circuit 122 is not able to deliver power for poweringmonitor 54. Rather, power is supplied from energy storage circuit 50. Inparticular, energy storage circuit 50 may supply a voltage Vstore tomonitor 54 and to input IN of boost converter 52A (FIG. 2). So long asthe voltage level of voltage Vstore is sufficient (i.e., above Vth4),energy storage circuit 50 can be used to power monitoring circuit 54 andvoltage regulator 52. During this time, monitor 54 may periodicallycheck the attachment status of electronic device 10. If voltage Vbusfalls below Vth3 during one of these checks as described in connectionwith FIG. 5, monitor 54 can return power converter circuit 122 and powerconverter 12 to active mode 106, as indicated by line 112. If monitor 54determines the voltage Vstore falls below Vth4 as described inconnection with FIG. 4, monitor 54 can momentarily activate powerconverter circuit 122 (active mode 118). In active mode 118, powerconverter circuit 122 is active and replenishes energy storage element50 (e.g., by recharging capacitor 80 path 66). Device 10 remainsdetached during the operations of mode 118.

After voltage Vstore has been restored (line segment 92 of FIG. 4),monitor 54 may return power converter circuit 122 and power converter 12to standby mode 144 to conserve power, as indicated by line 120.

If desired, Vaux can be provided at a different level (e.g., a levelthat is greater than the minimum operating voltage of device 10 or othersuch load but that is not greater than Vsec). In the example of FIGS. 4,5, 6, and 7, the use of a Vaux value that is greater than Vsec helpsfacilitate the detection of the attachment state of device 10 whenopening of switch SW2. In scenarios in which Vaux is not greater thanVsec, the presence of device 10 or other such loads may be detected bydetermining that the voltage Vbus has not fallen (e.g., Vbus has notfallen past a particular threshold voltage). Configurations in whichVaux is greater than Vsec are sometimes described herein as an example.This is, however, merely illustrative.

As shown in FIG. 8, the circuitry of system 8 may be incorporated intoall or part of an electronic device such as device 300. Device 300 maybe a portable computer, a handheld computing device, a desktop computer,consumer electronics equipment such as a television or stereo system, acomputer display, a game controller, or any other suitable electronicequipment. During normal operation, device 300 may be powered by thecircuitry of power converter 12. This allows the circuitry of device 300to be fully powered. Circuit components in device 300 are shownschematically as device circuit 210 in FIG. 8 and may include electroniccomponents such as user interface components (e.g., touch screens, touchpads, mice, keys, buttons, circuitry for receiving wireless usercommands such as infrared receiver circuitry that monitors signals fromremote controls, radio-frequency wireless communications circuitry thatmonitors user signals, processing and storage circuitry, sensors, etc.).

Energy storage circuit 50 may be charged during normal operation. Whenit is desired to conserve power, circuit 122 can be placed in areduced-power (standby) mode of operation. In standby, device circuit210 may await activity that indicates that device 300 should resumenormal operation. For example, device circuitry 210 may include infraredreceiver circuitry or other user input circuitry that monitors userinput activity or other suitable events. When a user supplies aninfrared command or other activity is detected by device circuit 210,the resulting behavior of device circuit 210 may cause the voltage online 72 to change. Monitor 54 can sense this change in voltage and mayissue a corresponding wake-up command to converter circuit 122 via path76. Monitor 54 can also periodically awaken converter circuit 122 toreplenish energy storage circuit 50, as described in connection withFIG. 1.

If desired, circuit 210 can inform monitor 54 that user input or othermonitored activity has been detected using other types of signalingschemes. Consider, as an example, the arrangement of FIG. 9. As shown inFIG. 9, electronic device 300 may have a power supply such as an AC-DCpower converter circuit 122 that charges energy storage and power(voltage) regulator circuit 302. Circuit 302 may be, for example, acircuit that includes an energy storage circuit such as energy storagecircuit 50 of FIG. 8 and that optionally includes a voltage regulator orother circuit that helps regulate the output of the energy storagecircuit when powering circuit 210.

Processor 304 may include storage and processing circuitry such as oneor more microprocessors and other control circuits (e.g., integratedcircuits, etc.). Processor 304 may be used in controlling the operationof device 300 and circuit 210.

During normal operation of device 300 of FIG. 9, power supply 122 maypower circuit 302, so the energy storage circuit 302 can be charged.Circuit 210 and processor 304 can be powered and can operate normally.When it is desired to conserve power, power supply 122 may be placed instandby (e.g., by processor 304, monitor 210, or other controlcircuitry). In standby, the energy storage circuit can be used to powercircuit 210 over path 306 and can be used to power monitor 54 over path308.

Circuit 210 can await user input such as an infrared remote controlcommand or other suitable event that indicates that device 300 should betaken out of standby mode. When such an event is detected, circuit 210can inform monitor 54 of the occurrence of the event by sending signalsover path 310. Path 310 can be an analog or digital path having one ormore associated lines for conveying communications between circuit 210and monitor 54.

Once monitor 54 determines that it is appropriate to wake up powersupply 122 to handle the user input command or other event, monitor 54can issue an appropriate wake-up control command for power supplycircuit 122 over path 76. Monitor 54 can also periodically wake up powersupply 122 when it is desired to replenish the energy storage circuit inenergy storage and power regulator circuit 302 via path 210.

FIG. 10 shows how device 300 may have multiple power supply circuits122. In the FIG. 10 example, device 300 has power supply circuit PS1 andpower supply circuit PS2. Power supply PS1 may be a high-power (primary)power supply that supplies tens or hundreds of watts of power, and powersupply PS2 may be a low-power (secondary) power supply that suppliesless power (e.g., ten or fewer watts of power). In standby, each supplymay consume only a fraction (e.g., 1-10%) of its active power capacity(as an example). These are merely illustrative examples. Primary supplyPS1 and secondary supply PS2 may have any suitable power supplycapacities if desired.

During normal operation, power supply PS1 may be in its active state andmay supply power to circuit 210, processor 304 and other components indevice 300. To conserve power, power supply PS1 may be placed in alow-power standby state when full power is not needed. Likewise, powersupply PS2 can be placed in standby to conserve power when activeoperation is not required. During standby, energy storage and voltageregulator circuit 302 may supply power to circuit 210, as described inconnection with FIG. 9. From time to time, the energy storage circuit incircuitry 302 may need to be replenished. As described in connectionwith the circuit of FIG. 1, monitor 54 can monitor the state of theenergy storage circuit. When replenishment is desired, monitor 54 canissue a replenishment control signal to power supply PS2 over path 314.In response, power supply PS2 can awaken from its standby state. Becausepower supply PS2 uses less energy than power supply PS1 and because theentire device 300 need not be powered during replenishment operationswith power supply PS2, the use of power supply PS2 to replenish theenergy storage circuit while power supply PS1 remains in standby canhelp conserve power.

If circuit 210 detects user input or other activity that indicates thatdevice 300 should enter its active state, circuit 210 can direct monitor54 to wake up power supply PS1 via path 316. Monitor 54 can also awakenpower supply PS2. If desired, circuit 210 can also use processor 304 toissue wakeup commands and other control commands. Processor 304 may, forexample, wake up power supply PS1 whenever user input is received withcircuit 210, whereas monitor 54 may be used in waking up power supplyPS2 (as an example).

An illustrative configuration for power adapter 12 such as power adapter12 of FIG. 1 is shown in FIG. 11. As shown in FIG. 11, the power adaptermay have a housing such as housing 318 in which circuitry such ascircuitry 12 of FIG. 1 may be mounted. Conductive prongs 320 may be usedto connect the power adapter to AC line power. Cable 322 may be used toroute output signals from adapter 12 to connector 324. Connector 324 maybe used to connect the power adapter to electronic device 10. Connector324 may be, for example, a 30-pin connector of the type that issometimes used in coupling music player and telephone devices tocomputers and power supplies. Connector 324 may, in general, have anysuitable number of contacts. The use of a 30-pin arrangement is merelyillustrative.

FIG. 12 shows another illustrative power adapter arrangement. In theconfiguration of FIG. 12, power adapter circuitry 12 of FIG. 1 ismounted within housing 326. Connector 328 in the FIG. 12 example may bea magnetic connector such as a MagSafe® connector from Apple Inc. ofCupertino, Calif. This type of connector uses magnet attraction to helpsecure connector 328 to a mating device. There may be, for example,magnets in portions 330 of connector 328. Plug type connectors may alsobe used in power adapter 12 if desired.

As shown in FIG. 13, energy storage circuit 50 may form part of AC-DCpower converter circuit 122. For example, converter circuit 122 may be aconverter circuit of the type that has a capacitor across its positiveand ground output lines (e.g., for filtering). In this type ofarrangement, energy for powering monitor 54 and for powering circuitssuch as circuit 210 (device 10 in the FIG. 13 example) may be storedwithin this filter capacitor, without the need for additional energystorage devices. In general, energy storage circuit 50 may be formedfrom any number of suitable components (capacitors, batteries, etc.) andthese components may form stand-alone circuits or may be combined intoother circuits in system 8 if desired. Examples such as the illustrativeconfiguration of FIGS. 1 and 13 are merely illustrative.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

1. An alternating-current (AC) to direct-current (DC) power converter towhich an electronic device may be connected when it is desired to powerthe electronic device with the power converter, the power convertercomprising: a power converter circuit that produces a DC voltage from anAC voltage; an output line to which the electronic device may beconnected; a switching circuit coupled between the power convertercircuit and the output line that periodically disconnects the powerconverter circuit from the output line; circuitry that determineswhether the electronic device is attached to the output line bymonitoring voltage levels on the output line when disconnecting thepower converter circuit from the output line with the switching circuit,wherein the power converter circuit comprises a control circuit andwherein the circuitry comprises a monitor that directs the controlcircuit to place the power converter circuit in a standby mode upondetermining that the electronic device has been detached from the outputline; an energy storage element that stores energy for powering themonitor when the power converter circuit is in the standby mode; avoltage regulator that receives an energy storage element voltage fromthe energy storage element and that provides a corresponding modifiedversion of the energy storage element voltage on a voltage regulatoroutput when the voltage regulator output is not loaded by the electronicdevice; and a path that electrically couples the voltage regulatoroutput to the output line.
 2. The power converter defined in claim 1wherein the monitor is configured to direct the control circuit tomomentarily place the power converter circuit in an active mode insteadof the standby mode when the monitor determines that the energy storageelement is to be replenished.
 3. The power converter defined in claim 2wherein the monitor is configured to direct the control circuit to placethe power converter circuit in the active mode upon determining that theelectronic device has been attached to the output line while operatingthe power converter circuit in the standby mode.
 4. The power converterdefined in claim 2 wherein the energy storage element comprises acapacitor.
 5. The power converter defined in claim 1 wherein themodified version of the energy storage element voltage is larger thanthe DC voltage produced by the power converter circuit.
 6. The powerconverter defined in claim 5 further comprising a current limitingcircuit interposed in the path that electrically couples the voltageregulator output to the output line.
 7. The power converter defined inclaim 5 wherein the voltage regulator comprises a DC-DC switched-modepower converter that produces the modified version of the energy storageelement voltage and wherein the modified version of the energy storageelement voltage is greater than the energy storage element voltage. 8.The power converter defined in claim 1 wherein the modified version ofthe energy storage element voltage is no larger than the DC voltageproduced by the power converter circuit.
 9. The power converter definedin claim 1 wherein the monitor is configured to direct the controlcircuit to place the power converter circuit in an active mode upondetermining that the electronic device has been attached to the outputline while operating the power converter circuit in the standby mode.10. An alternating-current (AC) to direct-current (DC) power converterto which an electronic device may be connected when it is desired topower the electronic device with the power converter, the powerconverter comprising: a switched-mode power converter circuit thatproduces a DC voltage from an AC voltage and that is operable in anactive mode in which the DC voltage is being produced and a standby modein which the DC voltage is not produced; an output line to which theelectronic device may be connected to receive an output line voltageequal to the DC voltage produced by the switched-mode power convertercircuit to power the electronic device; a switching circuit coupledbetween the power converter circuit and the output line; an energystorage element that produces an energy storage element voltage; avoltage regulator that receives the energy storage element voltage andthat has a voltage regulator output coupled to the output line; and amonitor that monitors the energy storage element voltage and the outputline voltage and that controls the switching circuit and theswitched-mode power converter circuit.
 11. The power converter definedin claim 10 wherein the monitor is configured to: momentarily open theswitching circuitry to isolate the output line from the switched-modepower converter circuit while the power converter circuit is in theactive mode; monitor whether the output line voltage rises while theoutput line is isolated from the switched-mode power converter circuit,indicating that the electronic device is detached from the powerconverter; and monitor whether the output line voltage falls while theoutput line is isolated from the switched-mode power converter circuit,indicating that the electronic device is attached to the powerconverter.
 12. The power converter defined in claim 11 wherein thevoltage regulator comprises a boost converter that provides a voltageregulator output voltage at the voltage regulator output that, when theelectronic device is detached from the output line and does not load theoutput line, has a voltage level that is larger than the DC voltageproduced by the switched-mode power converter circuit.
 13. The powerconverter defined in claim 12 wherein the voltage regulator furthercomprises a current limiting circuit that ensures that the output linevoltage drops when the electronic device is attached to the output linepower converter and the switched-mode power converter circuit isisolated from the output line by opening the switching circuit.
 14. Thepower converter defined in claim 10 wherein the monitor is configuredto: momentarily open the switching circuitry to isolate the output linefrom the switched-mode power converter circuit while the power convertercircuit is in the active mode; monitor whether the output line voltagedoes not fall past a given threshold while the output line is isolatedfrom the switched-mode power converter circuit, indicating that theelectronic device is detached from the power converter; and monitorwhether the output line voltage falls more than the given thresholdwhile the output line is isolated from the switched-mode power convertercircuit, indicating that the electronic device is attached to the powerconverter.
 15. A method for operating an alternating-current (AC) todirect-current (DC) power converter that has an output line to which anelectronic device may be connected when it is desired to power theelectronic device with the power converter, wherein the power converterhas a switched-mode power converter circuit that is operable in anactive mode in which a DC output voltage is produced and a standby modein which no DC output voltage is produced, wherein a switch isinterposed between an energy storage element and the output line, themethod comprising: momentarily isolating the switched-mode powerconverter circuit from the output line by opening the switch that isinterposed between the energy storage element and the output line;monitoring voltage levels on the output line; and in response to themonitored voltage levels, controlling whether the switched-mode powerconverter circuit operates in the active mode or the standby mode. 16.The method defined in claim 15 wherein monitoring the voltage levelscomprises determining whether the monitored voltage levels of the outputline have risen when momentarily isolating the switched-mode powerconverter circuit from the output line, indicating that the electronicdevice is detached from the output line, the method further comprising:placing the switched-mode power converter circuit in the standby modeupon determining that the electronic device has been detached from theoutput line.
 17. The method defined in claim 16 wherein monitoring thevoltage levels comprises determining whether the monitored voltagelevels of the output line have fallen when momentarily isolating theswitched-mode power converter circuit from the output line indicatingthat the electronic device has been attached to the output line, themethod further comprising: placing the switched-mode power convertercircuit in the active mode upon determining that the electronic devicehas been attached to the output line.
 18. The method defined in claim 17wherein monitoring the voltage levels comprises monitoring the voltagelevels using a monitor and the energy storage element comprises acapacitor, the method further comprising: powering the monitor with thecapacitor when the switched-mode power converter circuit is in thestandby mode.
 19. The method defined in claim 18 further comprising:momentarily placing the switched-mode power converter in the active modewhile the electronic device is detached from the output line tomomentarily produce the DC output voltage to recharge the capacitor. 20.Circuitry, comprising: a first power supply circuit; a second powersupply circuit; an energy storage circuit; a circuit that is powered bythe energy storage circuit when the first and second power supplycircuits are placed in standby state to conserve power; and a monitorcircuit that is configured to determine when the energy storage circuitis depleted and that is configured to wake up the second power supplycircuit to replenish the energy storage circuit without waking up thefirst power supply.
 21. The circuitry defined in claim 20 wherein thecircuit comprises circuitry that monitors user activity.
 22. Thecircuitry defined in claim 20 wherein the monitor circuit receivessignals from the circuit when the circuit detects activity and wakes upthe first power supply circuit to power the circuitry when the signalsfrom the circuit are received.
 23. The circuitry defined in claim 20wherein the monitor circuit receives signals from the circuit when thecircuit detects activity that indicates that the circuitry should beoperated in a normal operating mode and wherein the monitor circuit isconfigured to wake up the first power supply circuit to power thecircuitry in response to the signals from the circuit.
 24. The circuitrydefined in claim 20 wherein the energy storage circuit comprises acapacitor.
 25. The circuitry defined in claim 20 wherein the energystorage circuit comprises a capacitor and wherein the capacitor and thesecond power supply circuit form parts of an alternating current (AC) todirect current (DC) power converter circuit.
 26. An electronic device,comprising: a first switched-mode alternating current (AC) to directcurrent (DC) power supply circuit that operates in active and standbymodes; a second AC-to-DC power supply circuit that operates in activeand standby modes; an energy storage circuit that is at least sometimescharged by the second AC-to-DC power supply circuit while the firstswitched-mode AC-to-DC power supply circuit is in its standby mode; andat least one circuit that is powered using the energy storage circuitwhen the first switched-mode AC-to-DC power supply circuit and thesecond switched-mode AC-to-DC power supply circuit are operated in theirstandby modes.
 27. The electronic device defined in claim 26 furthercomprising a monitor circuit that communicates with the at least onecircuit and that is configured to change the first switched-modeAC-to-DC power supply circuit from its standby mode to its active modein response to signals received from the at least one circuit.